It sounds like you have had some back pain or other symptoms if you are having imaging studies of the lower back. In general, transitional vertebrae refer to a congenital or developmental anomaly in which a vertebrae has the characteristics of two different types of vertebrae. They typically occur at the junction between two different types of vertebrae, i.e. the cervicothoracic junction, the thoracolumbar junction, or the lumbosacral junction. Each type of vertebrae (cervical, thoracic, lumbar) has distinct physical characteristics such that a radiologist
can look at the bone itself and say exactly which type it is. In some people, during embryonic development the last or first vertebrae of a particular segment will not develop normally and will instead look like a hybrid of the two different vertebral types. Your radiology report suggests that your last lumbar vertebrae (L5) actually resembles a hybrid of lumbar and sacral anatomy.
Transitional vertebrae are not that unusual, and there are undoubtedly many people who have them but don't know that they do because they have not had imaging of their back. In your case, it sounds like you are having some kind of symptoms to have had this imaging done. The best thing to do in this case is to follow-up with the physician who ordered the study so that he or she can go over the results with you. Radiology studies can show many things that are not symptomatic--you need to go over this report with a physician who can go over your history and physical exam to see if any of your symptoms or exam findings may relate to this imaging finding.